In recent years, with the request for reduction of business trip expenses and a business trip time, call systems in which video conferences are performed through a communication network such as the Internet are in widespread use. In this call system, a video conference can be conducted such that image data and audio data are transmitted and received between a plurality of call terminals.
Besides a call system of a type in which image data and audio data are transmitted and received directly between call terminals, video conference systems of a type in which a plurality of relay devices are used to relay image data and audio data between a plurality of call terminals have been emerged (see Japanese Patent Application Laid-open No. 2008-227577).
However, in the video conference call system according to the related art, it is difficult to implement a call between call terminals using different call schemes. In other words, when call terminals differ in call scheme from each other, for example, when call terminals differ in at least one of a call control scheme (a call control protocol) for establishing or disconnecting a connection with a destination of a call and a coding scheme for converting image data and audio data into IP packets, there occurs a problem in that it is difficult to implement a call between the call terminals.